The disclosure relates generally to optical waveguide fibers, and more particularly to optical fibers exhibiting single polarization or polarization maintenance properties, and/or high birefringence.
Optical fiber has become a favorite medium for telecommunications due to its high capacity and immunity to electrical noise. Polarization maintaining (PM) fibers and single polarization (SP) fibers have been widely used to produce linear polarized output in optical systems. These fibers are useful for ultra-high speed transmission systems and are also utilized as couplers' fiber for use with, and connection to, optical components (e.g., lasers, EDFAs, optical instruments, interferometric sensors, and fiber gyroscopes). The single polarization fibers and polarization maintaining fibers can either be active, which means that they are rare earth doped in the fiber core, or passive, which means no rare earth dopants are involved. The polarization characteristic of single polarization fiber means that the fiber propagates one, and only one, of two orthogonally polarized polarizations within a single polarization band while suppressing the other polarization by dramatically increasing its transmission loss.
Polarization maintaining fibers (also referred to as a polarization retaining fibers) can maintain the input linear polarization on one of two orthogonally oriented axes. These fibers are not single polarization fibers. A common polarization maintaining fiber (referred to as PANDA PM fiber herein) includes, as shown in FIG. 1, a circular core 12′ surrounded by a pure silica cladding region 14′ with two stress-inducing regions 13′ (two stress rods) situated therein. Core 12′ and the cladding region 14′ are formed of conventional materials employed in the formation of optical fibers. The refractive index of the core material is greater than that of the cladding material. Current commercial PANDA type fibers, in order to obtain enough birefringence, have silica cladding and boron doped stress rods that that have greater than 20 wt % B2O3. During the draw process, because of silica cladding, the typical temperatures are grater than 1900° C., and these boron rods become quite fluid (low viscosity) relative to silica. During fiber draw the boron rods tend to “squirt”, which need to be corrected by adding complexity to the fiber draw processes, and by drawing fiber at slow speeds. The complexity and slow drawing makes this fiber relatively costly.
The two stress-inducing regions 13′ are formed of the same glass material, and thus have the same Coefficients of Thermal Expansion (CTE). The two stress-inducing regions 13′ also have composition different from core and cladding materials, and a CTE different from that of cladding material 14′. When such a fiber is drawn, the longitudinally-extending stress-inducing regions 13′ will shrink by a different amount than that of the cladding material, whereby fiber core 12′ will be put into a state of mechanical strain. Strain induced birefringence (otherwise referred to a stress-induced birefringence) is imparted in the fiber and thereby reduces coupling between the two orthogonally polarized fundamental modes.
No admission is made that any reference cited herein constitutes prior art. Applicant expressly reserves the right to challenge the accuracy and pertinence of any cited documents.